I've found that constantly syncing package explorer with editor causes package explorer view eventually to grow too long, especially with large projects. I've instead mapped a keyboard shortcut to sync package explorer with the editor. If you are using mylyn this is of course a smaller problem.

Key mappings are available at Window -> Preferences -> General -> Keys -> Show In (Show In Target Id: Package Explorer). Mine is ctrl-alt-left arrow, be welcome to copy.

Edit: In Luna Command name has changed a little. Instead of Show In (Show In Target Id: Package Explorer) command is now Show In (Package Explorer).

Press Alt+Shift+W while you’re in an editor. This pops up a menu with a number of choices. The choices will vary depending on what plugins you have installed. Select the option you want and press Enter.

The fastest way to select an option is to use the first letter of the option, eg. to go to the Package Explorer, press P (you may have to do this 2 or 3 times depending on how many other options start with P). The press Enter. You could also use the arrow keys.

Invoke "Show In" faster with a keyboard shortcut

If you frequently use one of the options (eg. Package Explorer), you can map a single keyboard shortcut to invoke it.

Go to Windows > Preferences > General > Keys.

Search for Show In. Eclipse will list a number of Show In options. In
our case we want Show In (Show In Target Id: Package Explorer).

Select the command you want, enter a key in Binding and you’re done.

Always link active source file with package explorer

If you like this feature, you can open the package editor and click on the double yellow arrows to always show the currently selected source file in the package explorer.